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Sidecar containers: what, why, and how

Introduction

In kubernetes a pod can contain multiple containers:

apiVersion: v1
kind: Pod
metadata:
  name: simple-pod
spec:
  containers:
  - name: container1
    image: nginx
  - name: container2
    image: busybox

Here we have 2 containers: container1 and container2.

When you have multiple containers in a pod we call them sidecar containers. Most of the time you have a "primary" container, the one with containing your application, and "secondary", hence the sidecar terminology.

All the containers of a given pod share the same network, and can share the same volumes.

Use cases

Sidecars are useful for containers that are tightly coupled. A good use case is when you migrate an app from one machine to containers. The application will have a lot of localhost or 127.0.0.1 hardcoded. So with sidecars you can work around this.

Another use case is to have sidecars helping the main container, like sending logs to a centralized system, sending the metrics to a specific system, doing SSL termination, etc.

Istio, the service mesh tool, installs a sidecar container to do its job: https://istio.io/docs/setup/kubernetes/additional-setup/sidecar-injection/

Exercices

Review and apply the file 01-sidecar.yml. Connect to the nginx container and look at the file system in /usr/share/nginx/html.

This exercice is taken from the official Kubernetes documentation.

Clean up

kubectl delete service,deployment,pod --all

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